1. Field of the Invention
The present invention relates to moving image processing for frame rate conversion and, more particularly, to a conversion process for performing convertion to a higher frame rate, for example, converting a 60 Hz image into a 120 Hz image.
2. Description of the Related Art
As a movie display device represented by a television receiver, a display device using a liquid crystal device (hold type) and a field emission type display device (impulse type) are known.
As a driving method of these display devices, a method of distributing a frame image into two sub-frames based on frequency components to take a motion blur measure for a hold type display device or a flicker measure for an impulse type display device is known.
As an example of a method of realizing double-speed driving of the hold type display device, a technique disclosed in patent reference 1 (Japanese Patent Laid-Open No. 2006-184896) is known. FIG. 10 shows a part of the circuit arrangement disclosed by patent reference 1. An LPF (low-pass filter) processor 104 generates a sub-frame image including only low frequency components based on an input frame image.
A difference detector 105 extracts, as high frequency components, a difference between the input frame image and the sub-frame image, which is generated by the LPF processor 104 and includes only low frequency components. A subsequent adder 199 adds the high frequency component image extracted by the difference detector 105 and the input frame image, thus obtaining a sub-frame image, high frequency components of which are emphasized.
A switching circuit 107 switches the sub-frame image including only low frequency components and that whose high frequency components are emphasized at cycles of 120 Hz, and outputs the selected image to the subsequent stage. Alternate displaying of the sub-frame image from which high frequency components are excluded and that whose high frequency components are emphasized is equivalent to reproduction of an original frame image when viewed at a time cycle of 60 Hz.
However, a frame image, which is observed as if two images were composited by alternately displaying the two sub-frame images, does not often become the same as the original frame image. This point will be described below with reference to FIGS. 11A to 11F.
FIG. 11A shows a waveform example of an input frame image. FIG. 11B shows a waveform as a result of processing of this input frame image by the LPF processor 104. FIG. 11C shows a waveform of a frame image output from the difference detector 105 when the input frame image having the waveform shown in FIG. 11A and the frame image having the waveform shown in FIG. 11B are input to the difference detector 105. Since this frame image is an image of high frequency components, its waveform assumes positive and negative values. FIG. 11D shows a waveform of a frame image output from the adder 199 when the frame image having the waveform shown in FIG. 11C and that having the waveform shown in FIG. 11A are input to the adder 199.
Theoretically, by alternately displaying the waveforms shown in FIGS. 11B and 11C at cycles of 120 Hz, an apparent waveform becomes the same as that shown in FIG. 11A. However, when a low luminance level portion in the waveform shown in FIG. 11A assumes a value equal to or close to zero, the waveform shown in FIG. 11D assumes a negative value. Since an image of the negative value cannot be displayed, the negative value is displayed as zero in practice, as shown in FIG. 11E. Then, an apparent composite waveform becomes a waveform shown in FIG. 11F, since the frame image having the waveform shown in FIG. 11B and that having the waveform shown in FIG. 11E are alternately displayed. When the frame image having such waveform is, for example, an image on which white characters are laid out on a black background, that image is perceived as an image on which the edges of these characters are blurred. In this manner, depending on the waveform of an input frame image, an image after the distribution processing cannot be seen as the same image as an original image, and is perceived as a degraded image, thus posing a problem.
In order to solve this problem caused by negative values generated upon division of sub-frame images, in patent reference 2 (Japanese Patent Application No. 2008-119059), a minimum value filtering unit 101 is arranged in front of the LPF processor 104 to apply minimum value filtering processing to the interior of a region to be processed by the LPF processor 104, as shown in FIG. 12. FIGS. 13A to 13F show waveforms of frame images obtained in respective stages by the apparatus having the arrangement shown in FIG. 12.
FIG. 13A shows a waveform of an input frame image. When the minimum value filtering unit 101 applies minimum value filtering processing to this input frame image, a frame image having a waveform shown in FIG. 13B is generated. When the LPF processor 104 applies low-pass filtering processing to the frame image having the waveform shown in FIG. 13B, a frame image having a waveform shown in FIG. 13C is generated. The frame image having this waveform is characterized in that gradient start points match the edges of the frame image having the waveform shown in FIG. 13A, since gradient regions due to the edges are shifted by a half interval length to the high signal value region side by the minimum value filtering processing.
The difference detector 105 generates a frame image having a waveform shown in FIG. 13D by subtracting the frame image having the waveform shown in FIG. 13C from that having the waveform shown in FIG. 13A. Since the frame image having the waveform shown in FIG. 13C assumes values lower than that having the waveform shown in FIG. 13A in every signal regions due to the minimum value filtering processing, the frame image having the waveform shown in FIG. 13D always assumes positive values.
The adder 199 generates a frame image having a waveform shown in FIG. 13E by adding the frame image having the waveform shown in FIG. 13D to that having the waveform shown in FIG. 13A.
Upon display, the frame image having the waveform shown in FIG. 13E and that having the waveform shown in FIG. 13C are temporally alternately displayed as sub-frame images. As a result, a frame image having a waveform shown in FIG. 13F (apparent frame image) can match that having the waveform shown in FIG. 13A (input frame image). In this way, the minimum value filtering processing has a negative value generation suppression function at the time of sub-frame division, thereby improving image quality.
However, this negative value generation suppression function of the minimum value filtering processing suffers a performance issue. That is, since this function depends on a picture on an image and pixel values selected by the minimum value filtering processing are too low, gradient regions of pixel values in edge regions are unwantedly extended by the low-pass filtering processing.
This issue will be explained below with reference to FIGS. 6A and 6B. FIGS. 6A and 6B are graphs both showing image information as linear signal waveforms, the abscissa plots image coordinates, and the ordinate plots pixel values. FIG. 6A shows image information when minimum value filtering processing is used, and FIG. 6B shows image information when no minimum value filtering processing is used. Both the pieces of image information shown in FIGS. 6A and 6B include edges near an image coordinate=10.
As shown in FIGS. 6A and 6B, as for a signal (SL) including only low frequency components, the gradient regions of pixel values obtained using the minimum value filtering processing are broader than those without using the minimum value filtering processing. When a picture remains unchanged temporally, since an apparent frame image obtained by compositing the two sub-frame images becomes the same as an original frame image, this phenomenon does not pose any problem in term of image quality. However, when a picture changes temporally, the above phenomenon imposes an adverse effect in terms of image quality, and a viewer perceives the gradient regions of the pixel values of the signal SL as a ghost in a direction opposite to a motion of an edge area. However, since this adverse effect is a tradeoff from the negative value generation problem at the time of sub-frame image division, it is permitted for negative value generation suppression.
A problem is that a minimum value of a signal including high frequency components (SH) assumes a value larger than zero when sub-frame image division is made using the minimum value filtering processing. Since the broader gradient regions of pixel values of the signal SL are a tradeoff from the negative value generation problem, the minimum value of the signal SH need not be larger than zero, and is most desirably set to be zero to minimize the gradient regions of pixel values of the signal SL if it is possible. However, it is difficult for the method and arrangement disclosed by patent reference 2 to implement it since patent reference 2 discloses only a function of selecting a minimum value of pixels in an interval.